1. Field of the Invention
The present invention relates to a logic circuit including a semiconductor element such as a transistor. The invention further relates to a semiconductor device including the logic circuit.
The semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics; a semiconductor circuit including the logic circuit and an electronic device such as an electro-optic device and a light-emitting display device each including the semiconductor circuit are all semiconductor devices.
2. Description of the Related Art
A general circuit including transistors formed by using a Si wafer or SOI (silicon on insulator) substrate can operate at a low voltage by microfabrication, and thus the circuit (and a semiconductor device including the circuit as well) has low power consumption.
Logic circuits for performing arithmetic processing operate when a power is supplied (the power supply is ON), and stop the operation when the power supply is stopped (the power supply is OFF). In the logic circuits, when the power supply is OFF, the contents of the processing performed right before the power supply is OFF are erased.
For example, as a logic circuit including a sequential circuit for storing logic states, such as a register circuit, a latch circuit, or a flip-flop circuit, there is a logic circuit in which a virtual power line and a virtual ground line are provided between a real power line and a real ground line, a p-channel transistor is provided between the real power line and the virtual power line, an n-channel transistor is provided between the real ground line and the virtual ground line, and the sequential circuit is provided between the virtual power line and the virtual ground line (see Patent Document 1). The logic circuit can operate in such a manner that the p-channel transistor and the n-channel transistor repeat turning on and turning off periodically so as to prevent a lack of data to be held in the logic circuit.
Further, the logic circuit can be configured by using a CMOS circuit including an n-channel transistor and a p-channel transistor, in which case, however, the off-state current is increased by microfabrication of a MOS transistor. Thus, there is a problem of increase in power consumption in a non-operating period (power consumption in a standby mode, hereinafter also referred to as standby power). For example, in a silicon MOS transistor having a channel length reduced to approximately 0.1 μm or less, even when the voltage applied to a gate when a potential of the source is used as a reference potential (this voltage is also referred to as Vgs) is lower than the threshold voltage, the value of current between the source and a drain cannot be zero. In this specification, current which flows between the source and the drain when the Vgs of the transistor is lower than the threshold voltage thereof is referred to as off-state current.
To prevent the increase of the standby power due to the off-state current, a technique using a switching transistor has been proposed (see Patent Document 2). The technique disclosed in Patent Document 2 is as follows: a switching transistor having low off-state current compared to a CMOS circuit is provided between a power supply and the CMOS circuit; the switching transistor is turned off when the CMOS circuit does not operate (when the CMOS circuit is not in operation) so that standby power is reduced.